Solid-state ignition systems are known in the art. U.S. Pat. Nos. 5,065,073 and 5,245,252, the disclosures of which are hereby incorporated by reference, teach, inter alia, that improved control over the performance of an ignition system can be achieved by incorporating a solid-state switch into an ignition output circuit. As taught by these patents, the ability of a solid-state switch to be triggered at a precise time allows an ignition system incorporating such a switch to achieve controlled spark rates. It also allows such a system to generate time-varying spark sequences. In addition, as explained in the above referenced patents, since a solid-state switch can be controlled independently of the voltage level of the ignition system's tank capacitor, an ignition system incorporating a solid-state switch can be used to deliver various amounts of energy by triggering the solid-state switch when a voltage associated with a desired energy transfer appears across the tank capacitor. This later effect cannot be achieved in older circuits using spark-gap switches since such switches fire only at a single voltage which is preset during manufacture of the spark-gap switch and will, thus, fire as soon as the voltage across the tank capacitor reaches the preset triggering level.
The '073 and '252 Patents also teach the desirability of waveshaping the current delivered into an igniter plug for a sparking event. For example, these patents teach that it is desirable to deliver a current to an igniter plug which initially increases at a low rate while ionizing the plug's gap and thereafter increases at a higher rate to sustain a spark across the ionized gap. Among other things, controlling the rise time of the current in this manner maximizes the life of the solid-state switch and the igniter plug by providing such components an opportunity to pass through their transition states before being taxed with a full, high energy pulse.
As mentioned above, prior art circuits such as those disclosed in the '073 and '252 Patents have achieved some degree of control over spark generation. However, prior art circuits such as these, while achieving many beneficial effects, have been somewhat constrained in their ability to control spark generation by certain physical limitations. For example, it is well known that the energy stored in an ignition circuit employing a tank capacitor is described by the formula: EQU Energy=1/2*Capacitance*(Voltage).sup.2
Thus, the energy delivered by such a circuit can be varied by changing either the charging voltage placed across the tank capacitor or the capacitance of the tank capacitor itself. There are, however, several practical limitations involved in varying these characteristics. For example, lowering the voltage levels used in the circuit requires a disproportionately large increase in the physical size of the capacitor used in the circuit to achieve similar energy levels. On the other hand, the available selection of capacitors, insulation materials, and solid-state switch components becomes limited at higher voltage levels.
The capacitance of prior art spark generating circuits is generally fixed when those circuits are constructed. In a circuit which uses a spark-gap switch the voltage is also fixed by the choice of the gap's breakdown voltage. Thus, traditional spark generating circuits are designed to deliver a predetermined energy level, but that energy level is thereafter unadjustable. In addition, prior art circuits have not attempted to control the plume shape of sparks generated at a spark generating device.
Ignition systems have been constructed for use as test apparatus wherein the user can manually vary the energy delivered by the system by physically connecting or disconnecting multiple capacitors to achieve various total capacitance and, thus, various total stored energy. However, from a safety standpoint, the high voltage and current levels in this part of the circuit makes physically switching capacitors in or out of the circuit somewhat impractical; usually requiring power-down and physical reconnection before sparking can continue. In addition, these systems have been limited to adjusting the total energy delivered and have not provided any spark shaping capabilities or real time control over the intensity and shape of the sparks generated.